1. Technical Field
This invention relates to speech coding, and more particularly, to a system that enhances the perceptual quality of digital processed speech.
2. Related Art
Speech synthesis is a complex process that often requires the transformation of voiced and unvoiced sounds into digital signals. To model sounds, sounds are sampled and encoded into a discrete sequence. The number of bits used to represent the sound can determine the perceptual quality of synthesized sound or speech. A poor quality replica can drown out voices with noise, lose clarity, or fail to capture the inflections, tone, pitch, or co-articulations that can create adjacent sounds.
In one technique of speech synthesis known as Code Excited Linear Predictive Coding (CELP), a sound track is sampled into a discrete waveform before being digitally processed. The discrete waveform is then analyzed according to certain select criteria. Criteria such as the degree of noise content and the degree of voice content can be used to model speech through linear functions in real and in delayed time. These linear functions can capture information and predict future waveforms.
The CELP coder structure can produce high quality reconstructed speech. However, coder quality can drop quickly when the coder""s bit rate is reduced. To maintain a high coder quality at a low bit rate, such as 4 Kbps, additional approaches must be explored. This invention is directed to providing an efficient coding system of voiced speech that accurately encodes and decodes the perceptually important features of voiced speech.
This invention is a system that enhances the perceptual quality of reconstructed speech. The system adds short-term enhancements to the spectral envelope of coded voice segments. The system includes a fixed codebook, an adaptive codebook, an enhancement circuit, and a summing circuit. The enhancement circuit interconnects an output of the fixed codebook to a summing circuit. The summing circuit adds an adaptive codebook contribution to a fixed codebook contribution. In another aspect, the enhancement circuit may be connected directly to a synthesis filter.
Other systems, methods, features, and advantages of the invention will be or will become apparent to one with skill in the art upon examination of the following figures and detailed description. It is intended that all such additional systems, methods, features and advantages be included within this description, be within the scope of the invention, and be protected by the accompanying claims.